As shown in FIGS. 1, 2, 3 and 4, conventional contrarotation type axial fan has a casing assembly structure including a first casing 110 and a second casing 120. The first casing 110 has a first surface 111 forming a plurality of first insertion grooves 112 and a plurality of second insertion grooves 113 thereon. The second insertion grooves 113 are located adjacent to the first insertion grooves 112 and extend to form a projective moving groove 113a. Each of the first insertion grooves 112 has a clasp-passing bore 112a and a clasp-moving bore 112b. The second casing 120 has a second surface 121 forming a plurality of clasps 122 and a plurality of protrusions 123 thereon. While assembling the first and second casings 110, 120, the clasps 122 of the second casing 120 are aligned with and inserted into the clasp-passing bores 112a of the first insertion grooves 112 on the first casing 110 and accommodated within the clasp-moving bores 112b of the first insertion grooves 112, which prevents the first and second casings 110, 120 from separation. Also, the protrusions 123 of the second casing 120 are accommodated within the projective moving grooves 113a of the second insertion grooves 113 for preventing the clasps 122 of the second casing 120 from slipping off the clasp-moving bores 112b. Accordingly, because it is necessary to form the second insertion grooves 113 and the projective moving grooves 113a on the first casing 110 and the protrusions 123 on the second casing 120, and align the protrusions 123 with the second insertion grooves 113 and the projective moving grooves 113a, mold-designing process becomes complicated and difficult.